The present invention relates generally to sheet feeding units and methods, and image readers, and more particularly to an automatic document feeder ("ADF") which sequentially feeds every one sheet from a pile of sheets, and an automatic sheet feeding method. The sheet feeding device and method according to the present invention are suitable for an ADF in an image reader, such as an image scanner, a copier, and a facsimile machine. Of course, the sheet is not limited to paper, but includes an OHP film and any other sheet material.
A document feeder for use with an image reader may be classified into a manual document feeder ("MDF") that requires a user to place every sheet to be read on a predetermined table, and an ADF that automatically feeds one by one each sheet when a user places one or more sheets to be read on a predetermined table. Unlike the MDF that requires a user to separate each sheet to be read, the ADF should be equipped with separator/feeder means for separating one sheet from a plurality of sheets to be read and for supplying it to the reading part.
A conventional ADF typically includes, as shown in FIG. 18, draw roller 2, separation roller 4, separation pad 6 arranged opposite to the separation roller 4, and tray part 8 opposite to the draw roller 2. Hereupon, FIG. 18 is a sectional view of essential part in the conventional document feeder 1. Plural sheets P are piled up on the tray part 8. The draw roller 2 is rotatable in an arrow direction and movable up and down as illustrated. The draw roller 2 contacts the uppermost sheet P in the pile and feeds one or more top sheets P between the separation roller 4 and the separation pad 6. The separation roller 4 rotates in an arrow direction as illustrated, separates one sheet P, in cooperation with the separation pad 6, and feeds it to the subsequent stage unit.
However, the conventional ADF has several drawbacks. First, left ends of the piled sheets P on the tray part 8, which arc out of alignment as shown in FIG. 18, often cause more than one sheets P to be fed at the same time. Due to the arrangement in which a position of the draw roller 2 is movable up and down according to the number of sheets P on the tray part 8 whereas positions of the separation roller 4 and the separation pad 6 are fixed, a height of the top sheet P in the pile to be introduced to the separation roller 4 differs according to the number of piled sheets P, thereby changing unstably the sheet separation condition according to the height of the sheets P on the tray part 8. In other words, the conventional ADF has a disadvantage in easily causing the double feed and/or jam due to the insufficient separation of sheets P.